There is a need to protect circuit elements from excessive current caused by transient load impedance or power supply surges. The commonly applied thermal fuse or electromagnetic circuit breaker opens the circuit at a predetermined current or current-time product. Such devices are slow acting and do not respond to fast acting transients. In addition, a fuse must be replaced after open circuiting and not all circuit breakers close automatically after transient conditions.
Other approaches to limiting current have led to complex or weighty circuit elements.